The invention relates generally to x-ray tubes and, more particularly, to a liquid metal bearing in an x-ray tube and a method of assembling same.
X-ray systems typically include an x-ray tube, a detector, and a bearing assembly to support the x-ray tube and the detector. In operation, an imaging table, on which an object is positioned, is located between the x-ray tube and the detector. The x-ray tube typically emits radiation, such as x-rays, toward the object. The radiation typically passes through the object on the imaging table and impinges on the detector. As radiation passes through the object, internal structures of the object cause spatial variances in the radiation received at the detector. The detector then emits data received, and the system translates the radiation variances into an image, which may be used to evaluate the internal structure of the object. One skilled in the art will recognize that the object may include, but is not limited to, a patient in a medical imaging procedure and an inanimate object as in, for instance, a package in a computed tomography (CT) package scanner.
X-ray tubes include a rotating anode structure for distributing the heat generated at a focal spot. The anode is typically rotated by an induction motor having a cylindrical rotor built into a cantilevered axle that supports a disc-shaped anode target and an iron stator structure with copper windings that surrounds an elongated neck of the x-ray tube. The rotor of the rotating anode assembly is driven by the stator. An x-ray tube cathode provides a focused electron beam that is accelerated across a cathode-to-anode vacuum gap and produces x-rays upon impact with the anode. Because of the high temperatures generated when the electron beam strikes the target, it is typically necessary to rotate the anode assembly at high rotational speed. This places stringent demands on the bearing assembly, which typically includes tool steel ball bearings and tool steel raceways positioned within the vacuum region, thereby requiring lubrication by a solid lubricant such as silver. Wear of the silver and loss thereof from the bearing contact region increases acoustic noise and slows the rotor during operation.
In addition, the operating conditions of newer generation x-ray tubes have become increasingly aggressive in terms of stresses because of G forces imposed by higher gantry speeds and higher anode run speeds. As a result, there is greater emphasis in finding bearing solutions for improved performance under the more stringent operating conditions.
A liquid metal bearing (i.e. a spiral groove bearing, or SGB) may be employed in lieu of ball bearings. Advantages of liquid metal bearings include a high load capability and a high heat transfer capability due to an increased amount of contact area as compared to a ball bearing. Advantages also include low acoustic noise operation. Gallium, indium, or tin alloys are typically used as the liquid metal, as they tend to be liquid at room temperature and have adequately low vapor pressure, at operating temperatures, to meet the rigorous high vacuum requirements of an x-ray tube.
However, liquid metals typically used in an SGB tend to be highly reactive and corrosive. The liquid metal of an SGB may react with a base metal that it contacts, thus consuming the liquid metal and shortening the life of the SGB. The rate of reaction is a function of temperature, and the temperature of an SGB tends to increase during operation—both because of high temperatures that occur during x-ray generation within the anode, and because of self-heating of the liquid metal. As such, the elevated operating temperature of the liquid metal may increase a loss rate of the liquid metal, leading to early life failure of the x-ray tube.
Therefore, it would be desirable to design an x-ray tube with an SGB having a reduced operating temperature therein.